1. Field of the Invention
The present invention relates to an image forming toner as well as an image forming method and apparatus for developing a latent image in electrophotographic methods, electrostatic recording methods, electrostatic printing methods and the like methods.
2. Discussion of the Background
Among various electrophotographic method, a typical electrophotographic method is:
(1) forming an electrostatic latent image on a photoreceptor by various means using a photoconductive material;
(2) developing the latent image using a toner to form a toner image;
(3) optionally transferring the toner image on a paper and the like; and
(4) fixing the toner image on the paper by heating, pressing or applying a solvent vapor to form a copy image.
As a method for developing an electrostatic latent image, there are two main methods. One is a liquid developing method using a developer in which various pigments and dyes are finely dispersed in an insulative organic liquid, and the other is a dry developing method such as a cascade method, a magnetic brush method and a powder cloud method and the like method using a toner in which a colorant such as carbon black is dispersed in a natural or synthetic resin. A dry developing method is widely used recently because of its easiness in handling.
As an fixing method in an electrophotographic method, a heat roller method is widely used because of its energy efficiency. In addition, recently, a heat energy given to a toner when fixing an image tends to become small for energy saving such as a low-temperature image fixation and a high-speed copy. Particularly, for saving energy, a reduction of total electric power consumption and CO2 emission is strongly demanded. Therefore, shortening of a standby time (i.e., warmup time of an apparatus) between the time when an image forming apparatus is turned and the time when an image can be formed, and minimization of an electric power for heating the fixing portion beforehand (i.e., preheating the fixing portion) to make the fixing portion quickly have a temperature at which an image can be fixed are demanded.
In the DSM (Demand-Side Management) program of International Energy Agency (IEA) in 1999, there is a technology procurement project for the next-generation copiers, in which the specification requirements are announced. For a copier having not less than 30 cpm, the above-mentioned standby time should not be longer than 10 sec. and the electric power consumption for the standby should not be greater than 10 to 30 W (which is different according to the copy speed). Compared with the conventional copiers, a dramatic reduction of total electric power consumption and CO2 emission is demanded. Particularly, in a high-speed image forming apparatus, a heating member is deprived of heat by a recording member, and an amount of heat for fixing an image is short. Therefore, further low-temperature fixability is necessary both for fixing devices and toners.
As an improvement of the fixing device, making the thickness of a roller which contacts the toner image supporting face not greater than 0.7 mm is attempted in order to increase the heat energy efficiency. The device significantly increases the energy efficiency and shortens the standby time. However, the mechanical strength of the roller becomes small, and the roller is deformed when a large load is applied thereto. Therefore, for a toner used for such an apparatus, low-temperature fixability which does not become a comparison with that of the conventional toner is required.
In addition, recently a need exists for high quality images, and sufficient image quality cannot be obtained by the conventional toners having a volume-average particle diameter of from 8 to 15 xcexcm. Therefore, a toner having a smaller particle diameter is required in respect of image quality as well.
As a toner having a small particle diameter, Japanese Patent No. 2763318 discloses a toner having a volume-average particle diameter of from 4 to 10 xcexcm, in which the quantity of the toner having a particle diameter of not greater than 5 xcexcm is 17 to 60% by number. However, particularly in the above-mentioned image forming apparatus having small total electric consumption by shortening the standby time, the low-temperature fixability is not sufficient. In addition, even the toner having a particle-diameter distribution so as to satisfy the low-temperature fixability in an image forming apparatus having a low facing-pressure fixing device has the following drawbacks:
(1) the toner is melted and adheres to the developing device and to the photoreceptor because the amount of the release agent, etc. present on the surface of the toner particles increases because the toner has a small particle diameter and a large surface area; and
(2) a non-image portion of the photoreceptor is developed with the toner. The reason is as follows. When cleaning the toner which is not transferred to the transfer member from the photoreceptor, the toner having such a small particle diameter (particularly a particle diameter of not greater than 3 xcexcm) passes the cleaning portion and contaminate the charger in case of a contact charging method. Therefore, a bias cannot be applied to the photoreceptor and the electric potential of the non-image portion is not increased, resulting in formation of background development.
Therefore, simply a toner having the particle-diameter distribution disclosed in Japanese Laid-Open Patent Publication No. 7-295283 is not satisfactory in the durability, and a further improvement is necessary to make a toner satisfying both the low-temperature fixability and the durability.
Accordingly, an object of the present invention is to provide an image forming toner having sufficient low-temperature fixability without irregularity and deterioration of the image density of the resultant image due to the toner fixedly adhered on a developing sleeve of an image forming apparatus even when repeatedly used.
Another object of the present invention is to provide an image forming method and apparatus using the toner.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an image forming toner characterized in that the toner includes at least a binder resin, a colorant and a release agent; a volume-average particle diameter of the toner measured by a Coulter counter is from 5 to 8 xcexcm; the content of the toner having a volume-average particle diameter of not greater than 5 xcexcm is 60 to 75% by number; and the content of the toner having a number-basis circle-equivalent particle diameter of from 0.6 to 3 xcexcm measured by a flow-type particle image analyzer is not greater than 25%.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.